Optical amplifiers are often used in optical communications to amplify a transmission signal. A signal being transmitted through a fiber optic system may suffer amplitude attenuation due to energy absorption, beam scattering and other processes during transmission. To compensate for such signal loss, optical amplification may be used to increase the amplitude of the signal leaving a transmitter or to increase the amplitude of a signal coming in to a receiver. Passive optical amplifiers are often used for such amplification.
Doped fiber amplifiers are one type of passive optical amplifier. Doped fiber amplifiers typically include a length of optical fiber that has been doped with one or more elements, such as rare earth elements (e.g., erbium). Such amplifiers amplify a transmission signal when the doped fiber receives optical energy from a pump source. Such amplification is produced by stimulated emission—the dopants in the doped fiber are stimulated to a higher energy state by receiving pump power in order to achieve a population inversion. As energy falls back to lower energy levels additional photons at the transmission signal wavelength and coherent therewith may be emitted. Usually the doped fiber responds most efficiently to one or more pumping wavelengths. A pump source is often chosen based on the wavelengths that are the most efficient pumping wavelengths for the doped fiber amplifier. Additionally, the amplification imparted to the transmission signal may have a gain curve with one or more peaks corresponding to specific transmission signal wavelengths. The doped fiber most efficiently and effectively amplifies signals with wavelengths near those peak wavelengths. The characteristics that define the output of a given doped fiber may include: gain per unit length, saturation power, noise figure, gain curve profile, and pump energy absorption.
Both long distance and metro area telecommunications systems include optical fiber, transceivers and passive optical amplifiers. However, metropolitan area networks (MANs) tend to have shorter distances between transceivers, are more cost sensitive than long distance telecommunications systems, and may be more tolerant to noise or bit error rates (BER). Embodiments of the present invention may be applicable to either type of system.